There has been considerable interest in using rare earth-doped optical fiber amplifiers to amplify optical signals used in communications systems and networks. These rare earth-doped optical fiber amplifiers are found to be cost effective, exhibit low-noise, provide relatively large bandwidth which is not polarization dependent, display substantially reduced crosstalk, and present low insertion losses at relevant operating wavelengths. As a result of their favorable characteristics, rare earth-doped optical fiber amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs), are replacing current optoelectronic regenerators in many optical lightwave communications systems and in particular, wavelength-division-multiplexed (WDM) optical communications systems and networks.
In an attempt to increase the capacity of these WDM optical communications systems and networks, it has been shown that it is generally desirable to have as many wavelength-division multiplexed (WDM) optical channels as possible within a given WDM system. As can be appreciated, broad band optical amplifiers are required to implement these "dense" WDM (DWDM) optical systems and networks.
With reference now to FIG. 1, there it is shown that the total possible gain spectrum for EDFAs is very wide. Unfortunately, however, the usable gain bandwidth for EDFAs is only about 10 nm, thereby limiting their utility for DWDM systems.
Of course, those skilled in the art will know that this gain bandwidth for EDFAs can be extended by approximately 40 nm, from around 1525 nm to 1565 nm through the use of Gain Equalization Filters (GEFs). See, e.g., A. K. Srivistava, J. B. Judkins. Y. Sun. L. Garrett, J. L. Zyskind, J. W. Sulhoff, C. Wolf, R. M. Derosier, A. H. Gnauck, R. W. Tkach, J. Zhou, R. P. Espindola, A. M. Vengsarkar, and A. R. Chraplyvy, "32.times.10 Gb/s WDM Transmission Over 640 km Using Broad Band, Gain-Flattened Erbium-Doped Silica Fiber Amplifiers," Proc. OFC, Dallas, Tex., pp. PD18, Feb. 16-21, 1997; Y. Sun. J. B. Judkins, A. K. Srivastava, L. Garrett, J. L. Zyskind, J. W. Sulhoff, C. Wolf, R. M. Derosier, A. H. Gnauck, R. W. Tkach, J. Zhou, R. P. Espindola, A. M. Vengsarkar, and A. R. Chraplyvy, "Transmission of 32 WDM 10 Gb/s Channels Using Broad Band, Gain-Flattened Erbium-Doped Silica Fiber Amplifiers," IEEE Photon Tech. Lett., 1997; and P. F. Wysocky, J. B. Judkins, R. P. Espindola, M. Andrejco, A. M. Vengsarkar, and K. Walker, "Erbium Doped fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating," Proc. OFC, Dallas, Tex., pp. PD2, Feb. 16-21, 1997. With further reference to FIG. 1, it can be seen that the gain for an EDFA drops sharply in the regions below 1525 nm and the regions above 1565 nm. Consequently it is impractical to further increase the gain bandwidth of EDFAs with GEFs since such an approach would require an unacceptably large amount of pump power and a correspondingly large number of GEFs to maintain an acceptably low noise figure.
Previous work has shown that significant optical gain can be obtained in the wavelength range between 1.57 and 1.60 .mu.m. See, for example, J. F. Massicott, J. R. Armitage, R. Wyatt, B. J. Ainslie, and S. P. Craig-Ryan, "High Gain, Broadband 1.6 .mu.m Er.sup.3+ Doped Silica Fibre Amplifier," Elec. Lett., Vol. 26, No. 14. pp. 1038-1039, September, 1990; J. F. Massicott, R. Wyatt, and B. J. Ainslie, "Low Noise Operation of Er.sup.3+ Doped Silica Fibre Amplifier Around 1.6 .mu.m," Elec. Lett., Vol. 26, No. 20, pp. 1645-1646, September, 1990. Additionally, new doping materials have been used to enhance erbium-doped fibers in the wavelength range between 1.53 and 1.56 .mu.m. In particular, Flouride EDF has been shown to provide additional gain and reports have indicated that Tellurite EDF shows great promise. See, e.g., A. Mori, Y. Ohishi, M. Yamada, H. Ono, Y. Nishida, K. Oikawa and S. Sudo, "1.5 .mu.m Broadband Amplification by Tellurite-Based EDFA's,", Proc. OFC, pp. PD1, Dallas, Tex., Feb. 16-21, 1997. Despite such promise however, the gain spectrum of such EDF is typically non-uniform and other important properties such as mechanical stability are poorly understood.
It is evident from this background then that alternative approaches to developing wide band optical amplifiers are required.